Many attempts have been made, many devices have been constructed, and many techniques have been used to plate articles that are to be exposed to environmental conditions such as high temperatures and corrosive atmospheres. The protection of such articles becomes especially critical when they are the parts used on the surfaces of components of spaced vehicles, aircraft, and other similar devices which may be exposed to the unfavorable environmental conditions because corrosion tends to damage or destroy the operating effectiveness of such devices and causes costly and tragic failures. It also has been desired to accurately and uniformly plate such workpieces with protective material without substantially changing their size, shape or dimensions and without weakening them.
There are many known and available plating apparatus and processes which have been used to protect parts from the effects of corrosion. However, most known processes create environmental problems that are becoming almost prohibitively expensive to solve, especially as local and national laws are becoming more strict. Those processes, such as ion vapor deposition, which do not create environmental problems like common cadmium plating proceses do have been unable to mass plate articles in sufficient quantities to be economically practical.
The plating apparatus described in the Steube patent referenced above, is an attempt to solve the mass plating problem primarily with ion vapor deposition. The basic process of ion vapor deposition is well known and was reported in the U.S.A. as nearly as November, 1963, by D. M. Maddox in a paper entitled "Film Deposition Using Accelerated Ions" in Sandia Corporation Development Report No. SC-DR-281-63.
In the known ion vapor deposition process, the workpiece to be plated is connected so that it is the cathode of a high voltage direct current glow discharge circuit which is operated within a vacuum chamber at low pressures in the presence of an inert gas. When the high voltage is applied, a glow discharge is established about the workpiece which is at the opposite potential from an evaporator boat containing the plating material. Some of the inert gas is ionized and accelerated toward the workpiece striking the workpiece with relatively high energy. This provides some cleaning of the workpiece surface. Once the inert gas bombardment has cleaned the surface of the workpiece to be plated, the temperature of the plating material is increased until it begins to evaporate. A portion of the evaporated plating material is ionized in the glow discharge region and accelerated toward the workpiece by the applied DC potential. Collisions between inert gas molecules and evaporant molecules also cause evaporant molecules to be directed toward the workpiece and to be deposited. The inert gas bombardment of the workpiece continues during the deposition and the rate of evaporation of the plating material from the boat is increased until the plating rate exceeds the rate at which material is being removed from the workpiece by the inert gas bombardment. The continued glow discharge cleaning of the workpiece and the high impact energies of the plating material ions give excellent coating adhesion and a good protective coating.
Although the presence of the inert gas increases the throwing power of the evaporated plating material so that it attempts to accumulate on all sides of an article to be plated, a more uniform plating layer having good adhesion characteristics can be acquired by use of a device such as that described in the above-referenced Steube patent. In that apparatus, a horizontally rotating barrel cathode is used to tumble the workpieces therein while they are being subjected to both cleaning and plating in the glow discharge. The tumbling action of the barrel, however, carries the workpieces a substantial distance up one side of the barrel, but still in the glow discharge region before they tumble back toward the bottom. This tends to move them in and out of the area of the barrel cathode closest to the evaporator boats. Heretofore the evaporator boats have been located directly below the rotating barrel. The evaporator boats are open on top and gravity is used to restrain the molten plating material in the boat. Any side positioning of the boat with respect to the barrel tends to enable the evaporated plating material to escape from the glow discharge. Therefore, it has been desired to create an apparatus wherein the workpieces remain in the desired area of the glow discharge while the source of evaporated plating material is in close proximity thereto.